The architecture of electronic circuits, and magnetic memory—both magnetic random-access memory and hard-disc memory—is essentially two-dimensional. A single bit is recorded in the free layer of a spin valve or in a thin patch of ferromagnetic material. With the relentless pressure to increase record density, the lateral dimensions of a bit are being pushed well below 100 nm, where the physical limits inherent in known magnetic materials will inevitably begin to make themselves felt. The simplest of these is the criterion for long-term thermal stability:KV/kT>60where K is an anisotropy constant of the ferromagnetic material, V is the volume of the magnetic bit, k is Boltzmann's constant and T is temperature. In the case of hard-disk recording, it has been possible to push back this superparamagnetic limit by ingenious magnetic engineering, but a radically different approach will be needed if magnetic memory is to be of use in the 2020s.
A way out is to somehow exploit the third dimension, storing the information vertically. Magnetic racetrack memory, Magnetic Domain-Wall Racetrack Memory as disclosed in S. S. Parkin, M. Hayashi and L. Thomas, “Magnetic Domain-Wall Racetrack Memory,” Science, vol. 320, no. 5873, pp. 190-194, 2008 uses this approach. A vertical shift register uses spin-transfer torque to drive magnetized magnetic domains (separated by domain walls).
Another idea is the magnetochiral permalloy stack as disclosed in PCT patent publication number WO/2010/055329, Cowburn, Russell P., entitled ‘Magnetic data storage using chiral soliton’. These solutions increase the storage per unit area by a factor of order 2n where 2 is the number of states in each bit, and n is the number of bits that can be stored vertically.
U.S. patent publication number US2009/0147562, assigned to Seagate Technology, discloses a compound magnetic data storage cell, applicable to spin-torque random access memory (ST-RAM), is disclosed. A magnetic data storage cell includes a magnetic storage element and two terminals communicatively connected to the magnetic storage element.
U.S. patent publication number US2007/0247901, Akinaga et al, discloses a mesoscopic magnetic body comprising a tabular ferromagnetic body whose planar shape has an axis of symmetry, but which is not symmetric in the direction perpendicular to the axis of symmetry, and wherein the magnetic body shows a circular single domain structure upon removal of the external parallel magnetic field.
It is an object to provide a new and improved magnetic structure for use in magnetic memory applications.